Method of producing sized particles of low-carbon ferrochromium

ABSTRACT

Method for producing sized particles of high-purity low-carbon ferrochromium by heating under vacuum an agglomerated mixture of finely divided high-carbon ferrochromium, oxidized ferrochromium containing sufficient oxygen to react with the carbon in the high-carbon ferrochromium, and a slag-forming material such as silica whereby a reaction product is obtained comprising lowcarbon ferrochromium in a matrix of slag. The agglomerated reaction product thus produced is crushed and the crushed material given a leaching treatment to remove the slag constituent.

ite States atet Chadwick [151 3,635,699 51 Jan. 18, 1972 METHOD OFPRODUCING SIZED PARTICLES OF LOW-CARBON FERROCHROMIUM [72] inventor:Cecil G. Chadwick, Lewiston, NY.

[73] Assignee: Union Carbide Corporation, New York,

[22] Filed: Dec. 22, 1969 [21] Appl. No.: 887,340

[52] US. C1 ..75/130.5, 75/0.5 BA, 75/24, 75/101 [51] int. Cl. ..C22c33/00, C22c 35/00, B22f9/00 {58] Field of Search ..75/130.5, 0.5 BA,24,101

[56] References Cited UNITED STATES PATENTS 2,206,711 7/1940 Udy..75/130.5 2,473,019 6/1949 Erasmus ..75/130 5 X 2,473,021 6/1949Spendelow, Jr. et a1. .........75/130.5 X 2,541,153 2/1951 Chadwick,.75/0.5 BA 3,459,542 8/1969 Knight ..75/i30.5

Primary Examiner--L. Dewayne Rutledge Assistant Examiner-G. K. WhiteAttorney-Paul A. Rose, Robert C. Cummings and Frederick J. McCarthy, Jr.

[ ABSTRACT 4 Claims, No Drawings METHOD OF PRODUCING SIZED PARTICLES OFLOW- CARBON FERROCHROMIUM This invention relates to the production offinely sized, highpurity low-carbon ferrochromium. More particularly thepresent invention is directed to a method for efficiently sizing lowcarbon ferrochromium produced by the reaction in vacuum of high carbonferrochromium and oxidized ferrochromium.

Low carbon ferrochromium is an important alloying material extensivelyused in the production of stainless steels. This material iscommercially produced by either of two general techniques, i.e., by theelectric furnace smelting of chrome ore and reaction with siliconreducing agents whereby lowcarbon ferrochrome is obtained in the form ofrelatively large dense lumps of metal, or by the vacuum furnacing ofagglomerated mixtures of high carbon ferrochrome and oxidant whereby theproduct is obtained in the form of briquettes.

While the bulk materials aforedescribed, i.e., lumps and briquettes, areeffective as addition agents in the manufacture of steel, there is adefinite need for a finely sized high-purity, low carbon ferrochrome foruse in the manufacture of welding rods and in chromizing and otheroperations.

it is therefore an object of the present invention to provide a methodfor readily producing sized particles of high-purity, low carbonferrochromium.

Other objects will be apparent from the following descriptions andclaims.

A method in accordance with the present invention broadly comprisesheating under vacuum conditions at elevated tern peratures anagglomerated mixture of finely divided high carbon ferrochromium, finelydivided oxidized ferrochromium, the oxygen content of the oxidizedferrochromium and a slagfonning material in the form of silica. The highcarbon ferrochrome and oxidized ferrochrome react to provide anagglomerated reaction product comprising fine particles of low carbonferrochromium in a matrix of slag; this material is readily crushableand is particulated to a desired size and leached to remove the slagconstituent. The ease of crushing to fine particle size is enchanced asthe silica addition is increased.

lt has been discovered, as part of the present invention that when amixture of high carbon ferrochromium and oxidized ferrochromium inproportions sufficient to decarbonize the high carbon ferrochromium andproduce low carbon ferrochromium, has incorporated therein from about0.5 to percent by weight of silica, the silica constituent does notparticipate in the decarbonizing reaction but instead fuses and forms aglassy matrix substantially surrounding grains of low carbonferrochromium reaction product. Low carbon ferrochromium in this form isreadily crushable by standard crushing techniques and the silica slagconstituent can be removed by leaching whereby a sized low carbonferrochromium material is obtained. Further, since the silicaslag-forming material does not participate in the decarbonizing reactionof a very high purity ferrochrome product is obtained. That is, thesilicon content of the product material is not increased and it has beenfound that the sulfur content of the product is also surprisinglyreduced. A silica content in an amount of from about 0.5 to 10 percentis effective in the present invention as previously noted; however fromabout 3 to 6 percent of silica is preferred since a remarkableimprovement in crushability is found at this level. Optimum crushabilityhas been found at a 4 percent silica content.

in the practice of the present invention a blended mixture of finelydivided high carbon ferrochromium, e.g., sized 300 mesh and finer andoxidized ferrochromium, e.g., also sized 300 mesh and finer is preparedby conventional techniques. The composition of the high carbonferrochromium ranges from about 65-75% Cr, 5-6% C, up to 2% Si, balanceFe and the oxidized high carbon ferrochromium has a composition of about50% Cr, 2630% 0 less than 0.5% C, balance Fe. The amount of oxidizedferrochromium in the mixture is such that the oxygen content thereof issufficient to react with the carbon of the high carbon ferrochromium. 1nthe mixture as aforedescribed is incorporated from about 0.5 to 10percent by weight of finely divided silica, e.g., from sized 60 to 300mesh and finer.

The resulting mix is then agglomerated for example in the form ofbriquettes. The agglomerating procedure is conven tional and can involveforming in a roll-type press using a suitable binder. The agglomeratesare heated under vacuum conditions at a temperature and for a timesufficient to effect reaction between the oxygen of the oxidizedferrochromium and the carbon of the high carbon ferrochromium to producelow carbon ferrochromium in the form of agglomerated grains of metal. ingeneral temperatures of from l,200 C. to l,400 C. and pressures of 100microns to 1,000 are satisfactory.

In the course of the reaction between the oxidant and high carbonferrochromium, the silica constituent remains chemically unaffected andfuses to provide a slag matrix for the grains of low carbonferrochromium in the agglomerated reaction product.

This material is then subjected to crushing to obtain a desired particlesize, and the crushed material is thereafter leached and the silicaconstituent removed. Suitable leaching materials are basic reactantssuch as NaOH and KOH.

The finely divided product obtained is of very low silicon content andis essentially silica-free, low carbon ferrochromi um material in thefinely divided state.

An important aspect of the leaching procedure is that a desliming anddecantation procedure is employed. That is, the crushed material isdigested in the leach solution and the very fine particles are floatedoff" and the leach solution decanted. With this procedure a practicallyslag-free final product is obtained. On the other hand, if ordinaryfiltration techniques are employed, the slag impurity content of thematerial is quite high, e.g., on the order of several percent.

The following examples will further illustrate the present invention.

EXAMPLE 1 Finely divided high carbon ferrochromium sized 300 mesh andfiner in the amount of parts having the following composition:

b by Weight Cr 7056 Ft: 20.98 C 5.42 5 0.074

and 25 parts of oxidized ferrochromium (52.51% Cr, 17.04% Fe, 26.00% 0sized 300 mesh and finer and 6 parts of silica also sized 300 mesh andfiner were thoroughly mixed with 5 parts of binder and 6 parts of waterand pressed into briquettes having dimensions of l 1%"Xl The briquetteswere dried and then heated in a vacuum furnace for 22 hours at atemperature in the range of l,000 to 1,385 C. at a pressure of about 300microns. The product obtained was in the form of briquettes with asilica matrix and had a composition as follows:

i: by Weight Cr 70.73 Fe 19.110 0 0.024 s 0.04:; Si 3.75 510, 4.63

The briquettes were then subjected to crushing in accordance with thefollowing procedure:

Twenty pounds of material sized 4 mesh were charged to a ball mill.After 1 hour 97 percent of the material was sized finer than 100 meshand there was no flattening or flaking of the particles as is the casewith a smelted low carbon ferrochrome material.

The material was leached with a 27 percent solution of NaOH by beingboiled overnight in a 30 percent pulp ratio and then washed bydeslirning and decantation. This was followed by filtering and washingto remove caustic and then drying at 105 C.

The analysis of the material before and after leaching is shown below:

Before Leaching After leaching The same procedure was followed as inexample I except that a lesser amount of silica, about 2 percent, wasincorporated in the mixture,

After milling to provide 97 percent of the material finer than 100 meshin the same manner as in example I, the product was leached in asolution of NaOH with the following results:

Before Leaching After Leaching Total Cr 69.82 70.69 Total Si 1.74 L22Metallic Si L06 0.86 Si(), 1.45 0.77 C 0.02 0.034 C30 028 0.17 O 1.40.83 MgO 0.22 0.10 N 0037 0.023 M 0, 0.53 0.36 S 0.02l 0.009

In addition to the foregoing the standard Bond Work Index was calculatedfor the materials of examples I and ll were calculated.

The index for the material of example I was found to be on the order of25 KWH/Ton and 39 KWH/T on for the material of example ll. The Worklndex represents the energy requested to size the material and as can beseen, with increased silica in the material, less energy is required. AWork lndex could not be calculated for smelted low carbon ferrochromiummaterial since this material could not be sized to mesh.

From the foregoing description it can be seen that the present inventionprovides a straightforward method of producing finely divided particlesof high purity, low carbon ferrochromium.

The mesh sizes referred to herein are United States Screen Series.

What is claimed is:

1. In the art of manufacturing low carbon ferrochromium alloy byreacting under vacuum at elevated temperatures an agglomerated reactionmixture of finely divided high carbon ferrochromium with finely dividedoxidized ferrochromium, the oxygen content of the oxidized ferrochromiumbeing sufficient to react with the carbon of the ferrochromium, theimprovement which comprises providing a minor proportion of silicamaterial in said reaction mixture whereby low carbon ferrochromium isproduced in a matrix of slag; crushing the low carbon ferrochromiummaterial and leaching the crushed material to remove the silica.

2. A method in accordance with claim 1 wherein the amount of silicaemployed is from about 0.5 percent to l0 percent by wei ht of thereaction mixture.

3. A met od in accordance with claim 1 wherein the silica IS in anamount of from 3 to 6 percent by weight of the reaction mixture.

4. A method in accordance with claim 1 wherein the amount of silica isin an amount of about 4 percent by weight of the reaction mixture.

2. A method in accordance with claim 1 wherein the amount of silicaemployed is from about 0.5 percent to 10 percent by weight of thereaction mixture.
 3. A method in accordance with claim 1 wherein thesilica is in an amount of from 3 to 6 percent by weight of the reactionmixture.
 4. A method in accordance with claim 1 wherein the amount ofsilica is in an amount of about 4 percent by weight of the reactionmixture.